Introduction

A Karst topography is formed when rock is dissolved, causing small fissures within the geologic structure. This soluble rock material is often limestone, dolomite, or gypsum. This caustic solution creates sinkholes in the soil surface and as it digs further into the rock, can create underground caverns and passageways. This allows precipitation and surface runoff to penetrate the soil horizons and fill subterranean caverns. This water can be held within the underground caverns, or exit though hydric tunnels. Karst springs are formed when water exits the subterranean matrix (though eroded tunnels) and forms surface pools.

Importance of Karst Systems

Karst systems host a wide variety of endemic species due to the unique nature of the habitat. With anthropogenic disturbance taking a heavy toll on the natural landscape, the potential for great harm to the karst topography exists. The microclimate of a karst means that any inputs to the system will remain isolated within that pocket and persists for a much greater time period. An introduction of toxic pollutants could potentially kill an entire species inhabiting this kind of system. Furthermore, with the expansion of the urban environment, the interaction between man and karst is increasing. Whether it be urban sprawl (depleting water levels and the species that reside with it) or ecotourism, humans are interaction more heavily with karst landscapes.

Physio-chemical Environment

Diagram of a Karst Formation.

The physio-chemical factors that are present within a karst system, mirror that of a cave ecosystem. Many of the same microbial communities (in addition to higher organisms) are present.

Chemical

The formation of the karst topography begins with the "carbon dioxide cascade." Rainfall absorbs CO₂ from the atmosphere during droplet formation, while it falls as precipitation, or as it moves through the soil horizons. This forms carbonic acid (H₂CO₃).

H₂O + CO₂--> H₂CO₃

As the acid comes in contact with the alkaline rock the two react, forming an aqueous solution that will further dissolve small channels within the soil. Slowly over decades channels will expand allowing larger volumes of water to pass through it. Due to the high solute concentrations within the water, karst systems are often found to be alkaline, having reacted with the calcareous environment and forming the karst.

Pollution

Karst systems are one of the greatest suppliers of aquifers, with nearly 25% of the drinking water (in the United States) flowing through a karst. The rapid recharge of the subterranean aquifers does mean that little filtration may occur with certain water sources. This may lead to potential pollution and contamination hazards.

Physical

The movement of water through any ecosystem affects the physical structure of that system. This is especially true for a karst. A karst is not only borne of the chemicals within water eroding away bedrock, but also the wearing of rock due to water movement.

Flowing water within a karst can hold a significant load of microorganisms and nutrients. The transportation of which can yield changes in the biological and chemical environments both at the source and sink regions of the karst watershed.

Biological interactions

In addition to the abundance of microbial organisms found in a karst, there are a plethora of higher organisms as well. Bats, birds, reptiles; animals associated with cavernous habitats, reside in karst systems. Consequently, there exists an relationship between the higher order inhabitants of these subterranean landscapes and the microorganisms that rely on the biological activities of these organisms.

It is often observed that subterranean waterways carry a lower load of dissolved organic matter. Therefore any input of nutrients, and organic substances re-charges the environment.

Microbial processes

The bacteria within the Karst landscape build and define the channels of the system. Through the cycling of carbon, carbon dioxide is utilized to build up, and break down calcium deposits.

Microbial communities acquire energy through the breaking down complex aromatic compounds; fixing volatile organic molecules, carbon dioxide acquisition, and nitrogen from the atmosphere; and also by oxidizing reduced metals within the cave rock.

Sulfur oxidation can occur in both chemoautotrophic and photoautotrophic organisms (where light is available).

Sulfur Oxidizing Organisms
Microorganism	Genera
Photoautotrophs
Purple Sulfur Bacteria	Chromatium
	Thiospirullum
	Rhodopseudomonas
Green Sulfur Bacteria	Chlorobium
Chemoautotrophs
Bacteria	Thiobacillus
	Thiosphaera
	Thiothrix
	Achromatium
Archaea	Sulfolobus
	Acidianus

Key Microorganisms

Bacteria

Phylum/Class	Genus
α-proteobacteria	Rhizobium
	Bradyrhizobium
	Sphingomonas
β-proteobacteria	Acidovorax
	Deefgea
	Duganella
	Iodobacter
	Janthinobacterium
	Massilia
	Pigmentiphaga
	Polaromonas
	Rhodoferax
γ-proteobacteria	Acinetobacter
	Aeromonas
	Buttiauxella
	Erwinia
	Escherichia
	Pantoea
	Pectobacterium
	Pseudomonas
	Rahnella
	Rheinheimera
	Serratia
	Shewanella
	Stenotrophomonas
	Yersinia
Bacteroidetes	Chryseobacterium
	Flavobacterium
	Hymenobacter
	Pedobacter
	Sphingobacterium
Firmicutes	Bacillus
	Sporosarcina
Actinobacteria	Arthrobacter
	Leifsonia
	Nocardia
	Rhodococcus

Algae

As with any open air body of water, karst springs host a number of different algal species including Cholorphyta and Bacillariophyceae. With the presence of sunlight, chlorophytic algae can germinate and grow.

Fungi

Ascomycetes:

Histoplasma capsulatum is a fungus found in karst systems and is often associated with the presence of bats (Myotis spp.). This fungus grows on the guano of bats on the floor of caverns and Karst systems.

Also found in karsts are fungi of the order of zygomycota

Current Research

1) One study analyzed the bacterial diversity and community composition in three karst cave system with different hydrologic and chemical traits. Specifically, they looked at the assemblage of microbes by the use of the 16S rRNA genes for identification. Accroding to the study, "Unexpectedly high microbial phylotype richness was found even in the two pools without groundwater contact and with low concentrations of organic carbon and total cell numbers." The conclusion of the analysis was that karst water provides a means of bacteria from a terrestrial ecosystem to a freshwater habitat via subsurface flow.

2) Research is also being done to determine the relationship between aerobic and anaerobic strains of bacteria within a single ecosystem. 681 species within a community were analyzed using 16s rRNA identification 35 genera were identified, but few were observed to dominate within the habitat consistently.

3) Further research has been done to show the relation between terrestrial peat bog microbial communities and subterranean aquifers. In studying a site in Italy, it was discovered that there exists a high rate of biological transfer through karst waterways between terrestrial habitats.